Jennifer Garner & Ben Affleck's Big Surprise: A Baby!

Jennifer Garner has convinced estranged husband Ben Affleck to save their marriage — with a baby! The Juno star showed off a tell-tale baby bump as she and Affleck, 43, returned from a four-day trip to romantic Paris — where they were spotted kissing in a hotel bar May 5 — and to London, where he is filming Justice League.

"Friends are expecting an announcement very soon that Jen is pregnant," a source told RadarOnline.com.

The couple announced a split last June after 10 years of marriage and three children, Violet, 10, Seraphina, 7, and Samuel, 4.

Friends said Garner, 44, got sick of the Batman v Superman star's drinking and gambling — and an alleged affair with their nanny was "the straw that broke the camel's back."

But they never filed divorce papers. "Jen has played Ben like a fiddle!" said a friend. "She was determined to make him 'walk the plank' over his wild ways. She believed he would come to his senses and they could save their marriage."

"She's always wanted another child with him. Even since they split, she's gushed publicly about how he is the love of her life," the source said.

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"First, she let him move back onto their property, living in a guest house. They've continued taking family vacations, spending all the important days in their children's lives together, even sleeping together."

"In many ways, Ben's been a lousy husband — but Jen still loves him and missed their steamy sex life!"

"When she gave birth to Samuel in 2012, she hoped things would change, since all Ben talked about was how much he wanted a son. But he fell back into his same old bad habits," the insider continued.

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Pals said it's been Garner's plan all along to keep him guessing, then hit him with the news that she's pregnant.